Aircraft and military ground vehicles are often required to operate under harsh environmental conditions. After prolonged exposure to such conditions, the windows of these vehicles, whether made from glass or plastic, tend to become scratched and pitted due to abrasion and the erosive effects of rain, hail, sand, dust, and the like. Plastic surfaces are also susceptible to damage from exposure to chemicals. As a result, visibility through these exposed surfaces becomes substantially impaired over time. To mitigate against the adverse effects of severe operating conditions, efforts have been made to devise effective protective coatings for various types of exposed surfaces. Unfortunately, these efforts have been met with limited success.
Tough, thermoset polyurethane coatings and films, sometimes referred to as "liners" exemplify one type of transparent coating system that have been used to protect glass and plastic surfaces from particle erosion and abrasion. Although effective at reducing damage caused by abrasion and erosion, they frequently are not sufficiently durable to resist the effects of weathering (i.e., long-term exposure to outdoor environmental conditions) or exposure to chemicals. Long-term outdoor exposure not only causes yellowing of the polyurethane material, but also promotes its degradation with respect to both transparency and mechanical durability. Conventional polyurethane coatings are also damaged by some organic chemicals and by strong acids, resulting in degraded vision quality.
A further drawback of using polyurethane as a protective coating material is that polyurethane surfaces are more difficult to clean, as compared to glass or conventional transparent plastic surfaces. This problem worsens with prolonged exposure to the environment. Specifically, as polyurethane surfaces weather, they become tacky. Dirt thus adheres to these tacky surfaces making cleaning increasingly difficult, causing further degradation of window vision quality. These limitations have thus far precluded the use of polyurethane coatings on transport aircraft windows.
Accordingly, a need exists for protective coatings that are both environmentally-resistant and transparent.